Picture frames with framed paintings or other framed artwork are often hung on a wall only to become crooked or unleveled. If initially hung straight and level, the picture frame often ends up moving due to vibrations of the wall to which it is hung and ends up hanging crooked.
Picture frames for much artwork are hung by a wire that stretches across the back of the frame. The wire is inserted onto a hook that is secured to a wall with one nail. The frame is then adjusted by sliding the wire over the hook until centered. This holds up the frame just fine but it may not stay level for long periods of time. Vibrations of a wall can cause the wire to slip on the nail so that the frame becomes crooked. The vibrations of the wall, for example, may be due to an earthquake, loud noises, or neighbors sharing a wall, floor or ceiling.
Another approach to hanging picture frames is to couple a straight sawtooth hanger to the back of the picture frame. The picture frame with the straight sawtooth hanger is then hung on a nail in the wall. The picture frame is then adjusted by laterally moving the straight sawtooth hanger over the nail until it pivots to the desired orientation. Friction between the base of the picture frame and the wall, if any, is supposed to keep the frame at a desired orientation, level with the floor. However, a picture frame with a single straight sawtooth hanger can still pivot about the nail and eventually may become crooked, such as from wall vibrations.
It is desirable to keep picture frames hung on a wall level.